Penetration is the medical term for the penis entering into the vagina. It's a good way term for talking about sex if one wants to seem mature. Even if you're a dirty little pervert, you'll sound like sex is more than just an emotionless action caused by hormones if you call it 'penetration' rather than 'fucking around,' 'doing it,' etc.

In sex the disappearence of the penis into the vaginal cavity and the rupturing the hymen.

Kiersten's examination in the ER showed that the source of her vaginal bleeding was penetration and the puncturing of the back wall of her vagina as the source of the blood. Kiersten spoke of falling on her bicycle handlebar, but the experienced personnel in the ER knew rape when they saw it. Who was the rider of the second bike that was found? Someone said Kiersten had been seenriding with Peter Goesinya.